The present invention relates to a system for injection molding large parts for example automotive components, door panels, instrument panels and the like which require very high flow rates of resin to fill the mold quickly.
Hot runner mold nozzles have been used in many molding applications. Typically a valve gated nozzle is used where high flow rates and a good gate mark are required. Large diameter open nozzles, i.e. nozzles having a diameter of 0.25", have been used in the past with the result that the gate mark produced thereby generally requires a post molding operation to remove the vestige. Valve gate arrangements are useful because they allow the use of gate diameters of 0.5" and larger while producing an acceptable gate mark.
Examples of valve gates are shown in U.S. Pat. Nos. 2,773,284 to Kelly, 3,488,810 to Gellert, 4,173,448 to Rees and 4,378,963 to Schouenberg. In these patents, the valve stem is either moved mechanically by a linkage or directly by means of a fluid operated piston and cylinder. Examples of valve gates where the valve stem is coaxial with the incoming flow channel and the moving mechanism is between the incoming channel and the stem are shown in U.S. Pat. Nos. 4,212,626 to Gellert, 4,378,963 to Schouenberg, and 4,449,915 to van den Brink.
All of the aforementioned patents use either hydraulic oil or compressed air to activate the piston and cylinder mechanisms which operate the valve stems. Care is taken to locate the piston and cylinder away from the heated component which contains the melt channel. By thermally insulating the piston and cylinder, the piston sealing ring is maintained at a temperature low enough to prevent it from failing.
Locating the piston, cylinder and seal in close proximity to the heated melt channel, which may be handling resin at 700.degree. F., requires a high temperature sealing device such as a cast iron piston ring to be used. Unfortunately, previous attempts to use such sealing devices have had an unacceptable high rate of leakage. Typically, compressed air has passed from one side of the seal to the other because the ring is split to permit assembly. The rate of leakage may still allow the piston to be activated if sufficient air flow is supplied. In the case of a multicavity mold using several valve gates, the volume of air required to overcome such leakage is generally unacceptable.
Hot runner systems have also been designed in many ways to accommodate the thermal expansion of their components. Very large systems must accommodate very large thermal expansions. U.S. Pat. No. 4,173,448 to Rees shows an example of how a hot runner manifold is allowed to expand and slide across the top surface of a fixed nozzle housing. The valve assembly is fixed to the manifold and slides with it across the backing plate surface. U.S. Pat. No. 4,682,945 to Schad explains this theory in some detail.
Another system for accommodating thermal expansion is shown in U.S. Pat. No. 4,378,963 to Schouenberg. In this system, the manifold is bolted to the mold cavity plate by a screw. Due to thermal expansion the manifold will slide over a fixed nozzle housing. This causes the screw to bend. When expansions are small this method is acceptable. Larger expansions on the other hand cause the screw to fail and/or the manifold/nozzle housing seal to leak.
Hot runner systems are also designed to accommodate injection forces. The nozzle housing and manifold in such systems typically must resist the force developed by the injection pressure acting on the projected area of the gate which tends to separate the nozzle from the cavity. In U.S. Pat. Nos. 4,173,448 to Rees and 4,682,945 to Schad, this force is counteracted by a back-up insulator which supports the manifold against the mold backplate. In U.S. Pat. No. 4,378,963 to Schoueberg, the screw bolting the manifold to the mold cavity plate counteracts this force.
It is an object of the present invention to provide a large nozzle hot runner system for injection molding large parts.
It is a further object of the present invention to provide a system as above having an improved piston sealing arrangement which permits the use of a cast iron sealing ring in a high temperature environment.
It is still a further object of the present invention to provide a system as above capable of accommodating both large manifold expansions and injection forces.
These and other objects and advantages will become apparent from the following description and drawings in which like reference numeral depict like elements.